1. Field of the Invention
The present invention relates to a thin-film magnetic oscillation element applied to, for example, wireless signal transmission, an element having a new function for communications.
2. Description of the Related Art
In magnetoresistive elements (MR elements), such as giant magnetoresistive (GMR) elements and tunneling magnetoresistive (TMR) elements, spin-polarized electrons flow when a current passes, and the magnetization direction of a free magnetic layer (i.e., the orientation of electron spin) varies depending on the number of spin-polarized electrons accumulated in the free magnetic layer. When an attempt is made to change the magnetization direction of a free magnetic layer arranged in a constant magnetic field, a torque acts on the electron spin so as to restore the electron spin to a stable orientation restricted by the magnetic field. When the electron spin is perturbed with a specific force, oscillation referred to as spin precession occurs.
In the case where spin precession is generated by spin-polarized electrons, a reduction in threshold current density required for oscillation (hereinafter, referred to as an “oscillation threshold current density”) has been one of the issues. Specifically, the oscillation threshold current density is required to be lower than 1.0×107 A/cm2.
To reduce the oscillation threshold current density, it is reported that a perpendicular magnetic film is used (Magnetics Japan Vol. 5, No. 4, 2010, pp. 184-190). According to this document, in the perpendicular magnetic film, a spin-transfer torque threshold current can be reduced by the demagnetizing field term, compared with an in-plane magnetic film. Thus, a threshold current of spin-torque oscillation, which is a precursor phenomenon of magnetization reversal due to a spin-transfer torque, should be reduced. In fact, when a magnetoresistive element in which a pinned magnetic layer and a free magnetic layer are perpendicular magnetic layers is used as a thin-film magnetic oscillation element, it is confirmed that the magnetoresistive element has an oscillation threshold current density of 5.0×106 A/cm2 (see Japanese Unexamined Patent Application Publication No. 2008-84879).
To achieve the spin-torque oscillation, the initial magnetic orientation of each of a pinned magnetic layer and a free magnetic layer is important. For example, with respect to the initial magnetization state, oscillation occurs easily when the magnetization directions of the layers are antiparallel rather than parallel (PHYSICAL REVIEW B 80, 174405(2009)).
As an example in which the initial magnetization directions of a pinned magnetic layer and a free magnetic layer are specified, a magnetoresistive sensor is reported (Japanese Unexamined Patent Application Publication No. 2003-229612), the magnetoresistive sensor including a magnetoresistive film which has a fixed magnetic layer (a pinned magnetic layer in the present invention) and which has a free magnetic layer with a nonmagnetic intermediate layer provided therebetween, the magnetization of the fixed magnetic layer being fixed in the plane of the layer, and the easy axis of magnetization of the free magnetic layer being oriented substantially perpendicularly to the plane of the layer; and a pair of electrode layers configured to allow a sense current to flow in a direction perpendicular to the plane of the magnetoresistive film.